1. Field of the Invention
The present invention relates to a method and an apparatus which can correctly predict the life of a rolling bearing having a specification that enables the basic dynamic load rating and the basic static load rating to being calculable, a rolling bearing selection apparatus using the life prediction apparatus, and a storage medium storing a program for life prediction.
2. Description of the Related Art
The basic rating life L10 of a rolling bearing is defined in JIS B1518: 1992, and usually calculated by the following expression:L10=(C/P)p  (1)where C is a basic dynamic load rating of the rolling bearing, P is a dynamic equivalent load which acts on the bearing, and p indicates a load index that is set to p=3 in the case of a ball bearing, and to p=10/3 in the case of a roller bearing. The basic rating life L10 indicates the life in the case where the reliability is 90%, usual materials are used, and the bearing is produced with a usual production quality and used under normal service condition.
By contrast, a corrected rating life Lna with respect to a reliability (100−n)% in the case where the failure probability is n%, special bearing characteristics, and specific service condition is given by the following expression:Lna=a1·a2·a3·L10  (2)where a1 is a reliability coefficient which is listed in Table 1 below, and which has a smaller value as the reliability is higher.
TABLE 1Reliability %Lnaa190L10a195L5a0.6296L4a0.5397L3a0.4498L2a0.3399L1a0.21
Moreover, a2 is a bearing characteristic coefficient which is used for correcting extension of the fatigue life due to improvement of materials. The coefficient is usually set to 1.0. When vacuum degassed bearing steel is used, the coefficient is set to 1.0 or more, and, when high cleanness steel is used, the coefficient has a larger value. The coefficient a3 is a service condition coefficient which is used for correcting influence on the lubrication condition, and, when a sufficient oil film thickness is expected, a3≧1 is set. By contrast, a3<1 is set when the viscosity of the lubricating oil in an oil contacting portion is excessively low, when the peripheral speed of a rolling element is very low, when the bearing temperature is high, or when a foreign material or water enters a lubricant.
In the related art example, correction to which the reliability, the bearing characteristics, and the service condition are added is performed on the basic rating life L10 of expression (1), whereby the accuracy of the prediction of the life of a rolling bearing can be improved. In the corrected rating life Lna, however, it is difficult to quantify the bearing characteristic coefficient a2 and the service condition coefficient a3, and particularly the degree of determining the service condition coefficient a3 is insufficient. Consequently, there arises a problem in that the basic dynamic rating life is dispersed. Therefore, it may be contemplated to handle a2×a3 as a single value. In this case, under usual lubrication condition, a2×a3=1 is set, and, when the viscosity of the lubricant is excessively low, the value is so small as that a2×a3=about 0.2. When the bearing is not inclined and the film thickness of the lubricating oil is sufficient at the operating temperature, a2×a3=2 may be employed. In this way, the product of the bearing characteristic coefficient a2 and the service condition coefficient a3 is varied in the range of 0.2 to 2.0, so that the life is changed ten times simply by the product of the coefficients. As a result, there is an unsolved problem in which the life prediction cannot be correctly performed.
As described in NSK Technical Journal (No. 655 (1993), pp. 17–24, FIG. 9), it is reported that dispersion of the actual bearing life L10 is very large. Namely, the actual bearing life is about twenty times the calculated life according to JIS in the case of super clean (using a two-stage filter), is substantially equal to the calculated life according to JIS in the case of mild contamination, and is about 1/7 to 1/25 of the calculated life according to JIS in the case of severe contamination. Therefore, it is very difficult to predict the life of an actual rolling bearing, and life prediction cannot be correctly performed.
As described in a report by Furumura, Murakami, Abe, et al. (ASTM STP 1195, J, JC. Hoo, Ed., 1993, pp. 199–210), depending on the cleanness of a material, the butterfly occurrence limit serving as an index indicating the rolling fatigue limit is varied, and, as compared with the occurrence limit of a material S (NSK standard material) which is at a contact surface pressure=1,850 MPa, the butterfly occurrence limit of a material L of lower cleanness is 1,100 MPa. With respect to a usual bearing material, when the contact surface pressure is 1,500 MPa or lower on the safe side in the case where a bearing is used under clean lubrication and ideal condition, the dynamic equivalent load at which peeling does not occur even after the number of repeated stresses reaches 1011 cycles can be considered as a fatigue limit load Pu.
As described in Proceedings of Japan Tribology Conference (Osaka, 1997-11, pp. 324–326), when an oil film parameter Λ[=hmin/√(hr12−hr22) where hr1 and hr2: mean square roughnesses of two contacting faces, and hmin: the minimum thickness of EHL oil film] serving as an index indicating lubrication condition is small (for example, Λ<3), the life is sometimes shortened to about 1/10 of that in the case where the oil film is sufficient, and sometimes not shortened. Therefore, an index indicating lubrication condition must be expressed not by the oil film parameter Λ, but by another parameter.